1. Field of the Invention
The present invention released to the field of proximal cable-less communication systems.
2. Related Applications
This application is related to U.S. patent application Ser. No. 07/477,680, filed Feb. 9, 1990, entitled "Parametrically Tuned Oscillator", U.S. patent application Ser. No. 07/477,677, filed Feb. 9, 1990, entitled "Hybrid Directional Coupler Circuit", U.S. patent application Ser. No. 07/477,583, filed Feb. 9, 1990, entitled "Improved Proximal Cable-less Communication System With Intentional Signal Path", U.S. patent application Ser. No. 07/477,632, filed Feb. 9, 1990, entitled "Method and Apparatus for Increased Receiver Sensitivity in a Proximal Cable-less Communication System", which are incorporated by reference.
3. Art Background
In the class of short-range (approximately 500 ft.) transceivers, such as cordless telephones or "walkie-talkies", basic super heterodyne techniques are used to receive an incoming signal and mix it with a local oscillator to provide an intermediate frequency (IF). The IF is detected and the resulting signal, such as voice, is amplified for end use. In transmission, the same transceiver unit accepts an input, such as voice, and modulates a carrier frequency. The resulting modulation envelope is amplified in the power amplifier for transmission.
Transmission of signals between two short-range transceivers still requires communication over a distance. To provide signal transfer across this distance, a nominal power level is required for transmission of intelligence. Such transmission would require a similar circuit as described above. When the distance is much shorter, communication may be achieved over lines or cable. Such direct physical connections are used because of simplicity and cost savings. However, physical connections significantly restrict the mobility of the users of the transceivers.
In U.S. Pat. No. 4,759,078, a proximal cable-less communications system using two receivers is described. A local oscillator of the first receiver is modulated to convey intelligence to a second receiver through the leakage radiation from the first local oscillator. Similarly, the local oscillator of the second receiver may also be modulated whereby the first receiver detects the leakage radiation from the local oscillator of the second receiver. The intermediate frequencies of the two receivers are set to frequencies such that the local oscillators provide the input signals to a mixer which generates signals at the intermediate frequency from which information is extracted. By proximately disposing the two receivers to each other, two-way transfer of information is achieved.
Using this approach, two way communication is achieved within a limited range without the need for complicated circuitry to provide the transmission of signals. Further, it permits a significant increase in user mobility.
This system is described by referring to the diagram of FIG. 1. Two way communication is achieved by tuning local oscillator (LO) 11 to a first frequency and LO 21 to a second frequency. Antenna 23 is tuned to receive the LO 11 frequency and antenna 13 is tuned to receive the LO 21 frequency. The intermediate frequency (IF) for both units 10 and 20 are determined by the difference of the frequencies of the two LOs, 11 and 21. The frequencies of the LOs 11 and 21 are set so that their difference is equal to the IF of both receiving systems.
Because of the proximity of the receivers to each other, antennae 13 and 23 are capable of receiving radiation from opposing LOs 21 and 11, respectively. Therefore, antenna 23 receives the first frequency radiation of LO 11 and mixes the signal with the second frequency from LO 21 in mixer 22 to provide an IF to Block 24. Correspondingly, antenna 13 receives the second frequency radiation from LO 21 and mixes the signal with the first frequency radiation from LO 11 in mixer 12 to provide the IF to block 14. By providing intelligence through the modulation of LO 11 and 21 signals, communication may be achieved between the units 10 and 20.
This system is inherently limited to modest power levels, namely the available local oscillator power, which clearly dictates the proximal range for the system. Furthermore, this system lends itself to applications inside buildings, as opposed to outdoor applications. As such, the performance is affected by the confines and obstructions within the building. For example, the propagation model for outside implementation might be close to free space or 1/r.sup.2 (where r is the radial distance), whereas in-building proximal systems may have a propagation model as restrictive as 1/r.sup.4. Thus, given the same source power, the effectiveness of the system may suffer a 30 dB loss at an approximate frequency of 1500 MHz over a 30 meter range. This loss of effectiveness is even more evident at UHF and higher, where path loss is more significant.
In addition, when the modulator (e.g., modulator 16 in receiver 10 of FIG. 1) is operating, and modulating the local oscillator (e.g. LO 11), and an incoming signal is detected, the signal received which is mixed with the modulated signal output by the local oscillator is somewhat distorted or contaminated by the modulation imposed on the oscillator. This distortion is referred to as sidetone. Although this feature is not always a problem, it is a hindrance when implementing digital systems and analog systems where sidetone is undesired, such as when the sidetone is significant to affect the value of the output information.